you can try F7 to try and toggle to the new screen but it probably won't work.

the only other thing to do is change your PRAM battery, which is the silver round watch battery inside under a clip. It's a lithium 3v you can buy anywhere for a couple of pounds/dollars. if you have had your mac on for a long time, i.e. don't turn it off at nights it may have run out.

otherwise it's something more serious like a kernel panic or the logic board. see other forum threads to find out about these.